Re: Cooperative Differential GPS
fyi, see http://www.trimble.com/gps/index.shtml for some good explanations of GPS works, including a section on Differential GPS regards Martin [EMAIL PROTECTED] wrote: Great to see someone who is interested in this as well! Thanks for bringing this up, as this is is a topic that I am very much interested in. We are just starting to explore this area, but the AGPS capabilities of the Neo 1973 was one reason that we went ahead and bought this. However, I do not know what AGPS support means in real terms. My naive understanding of this is that a cooperative differential GPS needs no external support. One just sits at a base station, and averages GPS readings until one knows the position of the base station to an arbitrary level of accuracy. After that, the reception of realtime GPS signals at the base station provides information about random errors, which I understand are largely due to atmospheric fluctuations. Corrections for these are derived from the difference between the well-known actual position of the base station, and the received realtime position. These can be broadcast through various means, e.g., an Internet DGPS server, and should be good for a few hundred km around the base station. The cooperative aspect of this comes in because of the benefits of multiple base stations. Being a non-expert the following should be taken with caution: What you describe here is - i think - refered to as poor man's DGPS because it would work with any cheap GPS receivers which only provide you with lat/lon coordinates. By just subtracting the lat/lon fluctuations of a known stationary GPS receiver nearby you do not gain any accuracy because you do not know exactly how the lat/lon was calculated and which errors influenced this calculation. As you said, the errors are due to atmospheric fluctuations but these vary wildly for each satellite in sight - that is why you need to have access to the raw data (ie pseudorange, carrier phase, doppler shift) per satellite. Consumer-grade GPS receivers will just use the pseudoranges from the satellites currently in sight and triangulate an estimated position. They don't tell you which satellites were used during this calculation and therefore you cannot gain any useful information for a receiver nearby. It is very improbable that two GPS receivers see _and_ use exactly the same satellites at a given moment, even if they are separated by just a few meters. You can actually do this experiment yourself (as I've done) and see that the lat/lon readings of two receivers in close proximity do not correlate much. However, with a stationary GPS receiver which provides you with the raw per satellite data you could extract the needed error information and publish it on some server. This could then in turn be used by a GPS receiver nearby (who also has to have access to the per satellite data) to calculate a much more precise location (errors might be as low as about a decimeter per kilometer distance to the reference GPS receiver). The good news is, that the GPS chip in the Neo FreeRunner could deliver the raw data but I am not sure if the firmware allows you access to the RXM-RAW message type cited in the Protocol Specification of the Antaris ATR0635. It would be great if someone could confirm that?! I think there are a bunch of people (including myself) which would be interested in such a Cooperative Differential GPS project for the FreeRunner (not to mention the great benefit for all users of an actual working CDGPS infrastructure) but for a start we would have to answer the question above. Best regards, -- beren ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Cooperative Differential GPS
Great to see someone who is interested in this as well! Thanks for bringing this up, as this is is a topic that I am very much interested in. We are just starting to explore this area, but the AGPS capabilities of the Neo 1973 was one reason that we went ahead and bought this. However, I do not know what AGPS support means in real terms. My naive understanding of this is that a cooperative differential GPS needs no external support. One just sits at a base station, and averages GPS readings until one knows the position of the base station to an arbitrary level of accuracy. After that, the reception of realtime GPS signals at the base station provides information about random errors, which I understand are largely due to atmospheric fluctuations. Corrections for these are derived from the difference between the well-known actual position of the base station, and the received realtime position. These can be broadcast through various means, e.g., an Internet DGPS server, and should be good for a few hundred km around the base station. The cooperative aspect of this comes in because of the benefits of multiple base stations. Being a non-expert the following should be taken with caution: What you describe here is - i think - refered to as poor man's DGPS because it would work with any cheap GPS receivers which only provide you with lat/lon coordinates. By just subtracting the lat/lon fluctuations of a known stationary GPS receiver nearby you do not gain any accuracy because you do not know exactly how the lat/lon was calculated and which errors influenced this calculation. As you said, the errors are due to atmospheric fluctuations but these vary wildly for each satellite in sight - that is why you need to have access to the raw data (ie pseudorange, carrier phase, doppler shift) per satellite. Consumer-grade GPS receivers will just use the pseudoranges from the satellites currently in sight and triangulate an estimated position. They don't tell you which satellites were used during this calculation and therefore you cannot gain any useful information for a receiver nearby. It is very improbable that two GPS receivers see _and_ use exactly the same satellites at a given moment, even if they are separated by just a few meters. You can actually do this experiment yourself (as I've done) and see that the lat/lon readings of two receivers in close proximity do not correlate much. However, with a stationary GPS receiver which provides you with the raw per satellite data you could extract the needed error information and publish it on some server. This could then in turn be used by a GPS receiver nearby (who also has to have access to the per satellite data) to calculate a much more precise location (errors might be as low as about a decimeter per kilometer distance to the reference GPS receiver). The good news is, that the GPS chip in the Neo FreeRunner could deliver the raw data but I am not sure if the firmware allows you access to the RXM-RAW message type cited in the Protocol Specification of the Antaris ATR0635. It would be great if someone could confirm that?! I think there are a bunch of people (including myself) which would be interested in such a Cooperative Differential GPS project for the FreeRunner (not to mention the great benefit for all users of an actual working CDGPS infrastructure) but for a start we would have to answer the question above. Best regards, -- beren ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
Re: Cooperative Differential GPS
(Please note that I am copying this message to the OpenStreetMap folk: http://openstreetmap.org as we will hopefully find more GPS/mapping experts there. Please edit the list of recipients if you need to.) On Thu, 27 Mar 2008 14:24:56 +0100 [EMAIL PROTECTED] wrote: Hi all! After hours of searching the web for information about the new Antaris GPS chip I am still not sure if it will be possible to use the raw data output of the ATR0635 (message type RXM-RAW) on the Neo FreeRunner. This message should, according to the protocol specification [1], contain information about the per-satellite errors necessary to build something like a cooperative differential GPS [2]. Such an application could - in theory - greatly improve the accuracy of the Neo-GPS and therefore open it up to a whole new range of use-cases. [...] Thanks for bringing this up, as this is is a topic that I am very much interested in. We are just starting to explore this area, but the AGPS capabilities of the Neo 1973 was one reason that we went ahead and bought this. However, I do not know what AGPS support means in real terms. My naive understanding of this is that a cooperative differential GPS needs no external support. One just sits at a base station, and averages GPS readings until one knows the position of the base station to an arbitrary level of accuracy. After that, the reception of realtime GPS signals at the base station provides information about random errors, which I understand are largely due to atmospheric fluctuations. Corrections for these are derived from the difference between the well-known actual position of the base station, and the received realtime position. These can be broadcast through various means, e.g., an Internet DGPS server, and should be good for a few hundred km around the base station. The cooperative aspect of this comes in because of the benefits of multiple base stations. I would love to be corrected on the above by someone who knows better, as this is little more than a surmise on how DGPS should work. Regards, Gora ___ Openmoko community mailing list community@lists.openmoko.org http://lists.openmoko.org/mailman/listinfo/community
